segwit: txout dust threshold calculation update

mruddy · sipa · commit 0acd1dc44967 · 2016-05-30T23:10:57.000+02:00
diff --git a/src/primitives/transaction.h b/src/primitives/transaction.h
@@ -169,15 +169,30 @@ class CTxOut
         // which has units satoshis-per-kilobyte.
         // If you'd pay more than 1/3 in fees
         // to spend something, then we consider it dust.
-        // A typical spendable txout is 34 bytes big, and will
+        // A typical spendable non-segwit txout is 34 bytes big, and will
         // need a CTxIn of at least 148 bytes to spend:
         // so dust is a spendable txout less than
-        // 546*minRelayTxFee/1000 (in satoshis)
+        // 546*minRelayTxFee/1000 (in satoshis).
+        // A typical spendable segwit txout is 31 bytes big, and will
+        // need a CTxIn of at least 67 bytes to spend:
+        // so dust is a spendable txout less than
+        // 294*minRelayTxFee/1000 (in satoshis).
         if (scriptPubKey.IsUnspendable())
             return 0;
 
-        size_t nSize = GetSerializeSize(SER_DISK,0)+148u;
-        return 3*minRelayTxFee.GetFee(nSize);
+        size_t nSize = GetSerializeSize(SER_DISK, 0);
+        int witnessversion = 0;
+        std::vector<unsigned char> witnessprogram;
+
+        if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
+            // sum the sizes of the parts of a transaction input
+            // with 75% segwit discount applied to the script size.
+            nSize += (32 + 4 + 1 + (107 / WITNESS_SCALE_FACTOR) + 4);
+        } else {
+            nSize += (32 + 4 + 1 + 107 + 4); // the 148 mentioned above
+        }
+
+        return 3 * minRelayTxFee.GetFee(nSize);
     }
 
     bool IsDust(const CFeeRate &minRelayTxFee) const
